The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine using a microcomputer, or more in particular to an air-fuel ratio control apparatus comprising means for compensating for the secular variations caused by contamination of an air-fuel ratio sensor or the like.
In conventional engine control systems using a microcomputer, data representing the engine operating conditions are collected by use of various sensors, an amount of a basic fuel supply is determined from these data, and the operation of the carburetor or the fuel injector is controlled through an actuator. Most of the engine control systems of this type comprises an air-fuel ratio control apparatus for operating the engine at a proper air-fuel ratio in order to improve fuel consumption rate and satisfy the exhaust gas control requirements.
The air-fuel ratio control apparatus specifically comprises an air-fuel ratio sensor represented by an oxygen sensor for accurate detection of the mixing ratio (air-fuel ratio) of the fuel and air supplied to the internal combustion engine, so that the air-fuel ratio is controlled to a proper value by a closed loop in response to an output of the air-fuel ratio sensor.
The air-fuel ratio sensor, however, which is mounted in the exhaust system of the internal combustion engine, is unavoidably contaminated with time by the exhaust gas after long engine operation. The detection accuracy of a contaminated air-fuel ratio sensor is deteriorated, thereby making it impossible to control the air-fuel ratio satisfactorily.
Conventionally, as disclosed in JP-A-58-57050, the atmospheric air is used as a known reference air-fuel ratio for calibrating the secular variations in the output characteristics of the air-fuel ratio sensor.
Specifically, in view of the fact that the output of the air-fuel ratio sensor reaches the maximum when the surroundings thereof are filled with the atmospheric air, the output value of the sensor which is surrounded by the atmospheric air and not yet contaminated in the initial stage of engine operation is used as a reference value. The output value of the sensor being contaminated by the usage of the engine is read when the sensor is surrounded by the atmospheric air. From the ratio between these two values, the compensation factor of the output characteristics of the air-fuel ratio sensor is calculated. The factor is multiplied with the output of the air-fuel ratio sensor thereby to obtain a correct output value of the air-fuel ratio sensor.
Whether the air-fuel ratio sensor is surrounded by the atmospheric air is determined by detecting whether the engine is in a fuel cut state such as a deceleration state or a non-started state or not. Specifically, when the engine is in a deceleration state, for example, if the throttle valve is closed and the engine speed is reduced below a predetermined level, it is decided that fuel has been cut, and assuming that the surroundings of the air-fuel ratio sensor is filled with the atmospheric air upon a lapse of a predetermined length of time later after the decision. Thus, the output value of the air-fuel ratio sensor after the lapse of the predetermined time is read thereby to calculate the above-mentioned compensation factor.
Depending on the operating conditions before deceleration, however, even after the lapse of the above-mentioned predetermined length of time, fuel may remain attached on the interior of the intake manifold or the mixture gas may exist in the exhaust port, with the result that the output value of the air-fuel ratio sensor may not represent a value when the surroundings of the air-fuel ratio sensor are filled with the atmospheric air. Therefore, the desired maximum value of the air-fuel ratio may not be obtained. If the output characteristics of the air-fuel ratio sensor are calibrated on the basis of this inaccurate output maximum value thereof, the air-fuel ratio is not controlled properly. One method of preventing this inaccurate detection of the maximum value of the output of the air-fuel ratio sensor is to set the above-mentioned predetermined time sufficiently long. Nevertheless, if the predetermined time is excessively long, the maximum output value of the air-fuel ratio sensor is less likely to be detected under the above-mentioned conditions, and therefore there are fewer chances of calibrating the output characteristics. Thus, it makes it difficult to detect the output of the air-fuel ratio sensor accurately.
Before the engine is started, on the other hand, when the ignition switch is turned on but the engine speed is zero, it is decided that the exhaust port is filled with the atmospheric air, and the output of the air-fuel ratio sensor at this time is read. In the case where the ignition switch is turned on immediately after the engine stops, however, the exhaust gas or the like may still remain in the exhaust port and it is difficult to detect the maximum output value of the air-fuel ratio sensor, thus making accurate calibration of the output characteristics thereof impossible.
Further, since a lean sensor is used as the air-fuel ratio sensor in the conventional system, the closed loop control of the air-fuel ratio is impossible in the rich mixture region of the air-fuel ratio.